1. Field of the Invention
The present invention relates generally to methods and apparatus for seismic exploration, and, more particularly, relates to a method and apparatus for controlling the vibrating force imparting to the ground in such prospecting.
2. Description of the Prior Art
Hydraulic vibrators are commonly used in seismic prospecting to impart seismic waves into the earth's crust. The vibrator causes a baseplate loaded by an isolated mass, normally the carrier vehicle, to vibrate against the earth's surface. If the vibrating force is to great, i.e. exceeds the sum of the masses of the reaction mass, baseplate, and holddown force, the baseplate lifts off the ground, i.e., "decouples", which is an undersirable result. When the baseplate decouples and then subsequently engages the earth's surface undesired noise sources, which are caused by the baseplate slapping the ground, interfere with the other seismic waves that are being measured by the geophones.
A desired frequency range of a seismic hydraulic vibrator is generally considered to be from 5 to about 200 Hz. The maximum hydraulic force that seismic vibrators generate is typically about 50,000 lbs. Seismic energy sources with this force and frequency range present some conflicting design requirements. For example, the size and hence the weight of the baseplate should be larger for lower frequencies and smaller for higher frequencies. The size and weight of the baseplate affects the frequency at which maximum energy is coupled to the earth, as well as the efficiency of such coupling. Other conflicts appear when considering the stiffness and weight of the main piston/baseplate structure. The mass of the structure should be small, however, the structure should preferably be stiff enough to avoid mechanical resonances below about 2.5 times the maximum operating frequency. The main piston stroke length should generally be as short as possible and should trap a minimal volume of hydraulic fluid within the cylinder. The main piston stroke length, however, must still be long enough to accommodate the lowest frequency. In many vibrators, a servo hydraulic valve is utilized to drive the vibrator piston of the vibrator. This servo hydraulic valve must be large enough for the lowest frequency and yet fast enough for the highest frequency.
Differences in earth surface conditions (earth impedance) influence a natural resonant frequency between the earth and the vibrator. The natural resonant frequency of the vibrator/earth system typically occurs in a frequency range of 20 to 45 Hz. Optimal earth coupling of most hydraulic vibrators appears to be achieved in this range.
In a vibrator wherein a servo hydraulic valve is used to drive the main piston, the valve is often precisely controlled by a feedback amplifier so that the position of the valve spool corresponds to the input voltage of the amplifier. This input voltage is typically a constant amplitude sweep frequency. Servo control stops at the output of this valve, and the main hydraulic force generator operates without feedback control. Therefore, the output response of the hydraulic force generator is determined by the natural response characteristics of the earth/hydraulic generator system and is subject to undesirable variations.
The typical force response characteristics of a seismic vibrator (measured at the main hydraulic piston) are as follows: beginning at 5 Hz, the force is about 12 decibels below normal output, and increases at the rate of 6 decibels per octave up to the earth/hydraulic system resonant frequency, for example, 25 Hz. From system resonance to the maximum frequency response of the servo valve (80 to 150 Hz), the response is relatively flat at 0 decibels. Above the maximum of level frequency of level valve amplitude response, the response falls off at a rate of 6 decibels to 12 decibels per octave. At system resonance, a response peaking of 4 decibels to 10 decibels is typically observed. Thus, force variations of 16 decibels to 22 decibels have occurred over the normal frequency range of the vibrator.
Various attempts have been made to devise a feedback loop that will accurately minimize these vibratory variations. In U.S. Pat. No. 3,208,550 to Castanet, there is disclosed a vibrator with an automatic controlling system in which the force imparted to the earth is kept proportional to a controlling electrical signal. Mifsud, in U.S. Pat. No. 4,049,077, discloses generating a feedback signal equal to the instantaneous amplitude of either the baseplate velocity or displacement and utilizing one of these feedback signals to control the instantaneous motion of the baseplate. The system disclosed in Mifsud requires a determination in advance, for a particular location, of the maximum velocity or displacement amplitude that can be generated over the frequency range of interest without decoupling the baseplate from the ground. Rickenbacker, in U.S. Pat. No. 4,184,144, discloses using a signal related to the peak force transmitted by the baseplate in a controlled network, to either manually or automatically vary the amplutide of the input signal to the vibrator.
An automatic gain control system such as Rickenbacker's system has the disadvantages that the feedback loop may be not fast or smooth enough, particularly at low frequencies, and also will not improve the system phase response.
Another method of force amplitude correction is the frequency equalizer method. In this method, the equalizer circuit generates a transfer function which is the inverse of the system transfer function. Combining these two transfer functions produces an overall transfer function which is uniform over the operating range. With this method, however, accuracy of the amplitude and phase corrections may be no more accurate than the accuracy of the match between the two complementing transfer functions. Accordingly, the present invention provides a new method and apparatus for controlling the operation of a hydraulic vibrator offering optimal speed and frequency response as well as optimal accuracy of the vibrator amplitude.